Transistors and other semiconductor components are conventionally assembled in so-called packages in which the various zones of each silicon body are soldered to respective external conductors. Generally, one major body surface (referred to hereinafter as the front surface) is provided with a plurality of metallic pads, overlying disjointed contact areas of that surface, whereas the opposite surface (referred to hereinafter as the rear surface) is of essentially uniform character and is completely metallized. Low-melting alloys, e.g. of lead or tin, are suitable bonding agents for soldering the various pads of the front surface to individual terminal leads and the metallic coating of the rear surface to a conductive substrate, e.g. a copper plate serving as a heat sink.
Especially for power amplifiers and similar devices designed to handle currents above 2 amps, aluminum is an excellent contact material forming low-ohmic junctions with a doped silicon substrate of N-type or P-type conductivity having the usual impurity concentrations. Being an inexpensive metal and easy to apply, aluminum is advantageously used on the frontal contact areas of a silicon body. This metal, however, is not bondable by a low-melting alloy of the type referred to.